Giant otters feeding on caiman: evidence for an expanded

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Giant otters feeding on caiman: evidence for an
expanded trophic niche of recovering populations
Carolina Ribas
a b
c
a
, Gabriel Damasceno , William Magnusson , Caroline Leuchtenberger
& Guilherme Mourão
b
a
Instituto Nacional de Pesquisas da Amazônia – INPA Manaus, AM, Brazil
b
Embrapa Pantanal, Brazilian Agricultural Research Corporation, Corumbá, MS, Brazil
c
CCG Productions, Rio de Janeiro, RJ, Brazil
Available online: 13 Mar 2012
To cite this article: Carolina Ribas, Gabriel Damasceno, William Magnusson, Caroline Leuchtenberger & Guilherme Mourão
(2012): Giant otters feeding on caiman: evidence for an expanded trophic niche of recovering populations, Studies on
Neotropical Fauna and Environment, 47:1, 19-23
To link to this article: http://dx.doi.org/10.1080/01650521.2012.662795
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a b
Studies on Neotropical Fauna and Environment
Vol. 47, No. 1, April 2012, 19–23
SHORT COMMUNICATION
Giant otters feeding on caiman: evidence for an expanded trophic niche of recovering populations
Carolina Ribasa,b*, Gabriel Damascenoc , William Magnussona , Caroline Leuchtenbergera,b
& Guilherme Mourãob
a
Instituto Nacional de Pesquisas da Amazônia – INPA Manaus, AM, Brazil; b Embrapa Pantanal, Brazilian Agricultural
Research Corporation, Corumbá, MS, Brazil; c CCG Productions, Rio de Janeiro, RJ, Brazil
(Received 2 November 2011; accepted 30 January 2012)
Downloaded by [UNESP] at 11:40 04 April 2012
As water along dirt roads in the Pantanal floodplains diminishes, aquatic fauna becomes restricted to shallow
pools. At the end of the 2009 dry season, we filmed giant otters living in pools predating on yacare caimans.
Such predation has not been recorded in giant otters inhabiting the Pantanal. Individual otters captured sub-adult
caimans. The otters did not share the prey, but conspecifics stole it after conflicts. Caiman predation could be
related to resource scarcity in these marginal environments. Information on diet and interactions of endangered
populations may underestimate their trophic niche when they recover from over-hunting and expand into suboptimal habitat.
À medida que as águas ao longo das estradas de terra na planície pantaneira diminuem, a fauna aquática se torna
restrita às poças rasas do entorno. No final da estação seca de 2009, nós filmamos ariranhas predando jacarés
em baías temporárias. Este tipo de predação não foi descrito antes para ariranhas do Pantanal. As ariranhas
capturaram jacarés sub-adultos, individualmente. A presa não foi dividida voluntariamente, mas foi roubada por
outros indivíduos do grupo, após conflitos. Esta predação pode estar relacionada com a escassez de alimento
nestes ambientes marginais. Informações de dieta e interações em populações ameaçadas talvez subestimem seu
nicho trófico quando estas se recuperam, após período sem caça, e expandem para ambientes sub-ótimos.
Keywords: Ariranha; Pteronura brasiliensis; predation; Caiman crocodilus yacare; Pantanal floodplains; Brazil
Introduction
Giant otters (Pteronura brasiliensis) are large social
semi-aquatic mustelids endemic to South America.
Predominantly piscivores throughout the geographical distribution of the species, their preferred fish
are from the orders Characiformes, Perciformes and
Siluriformes (Duplaix 1980; Laidler 1984; Rosas et al.
1999; Rosas-Ribeiro 2009). Giant otters take quite
large fish (> 30 cm), especially slow-moving species,
according to their availability and ease of capture
(Duplaix 1980; Schweizer 1992; Kruuk 2006). While
some reports indicate that they consume varied foods,
such as crustaceans, mollusks, reptiles, amphibians,
water birds, and mammals, these represent less than
1% of their diet (Duplaix 1980; Schweizer 1992; Rosas
et al. 1999; Rosas-Ribeiro 2009; Cabral et al. 2010;
Silva 2010).
Giant otters suffered a drastic reduction in distribution as a consequence of excessive hunting to supply
the fur trade during the 1950s and 1960s (Carter
& Rosas 1997) and habitat destruction (Schenck
1999). The decrease in demand on the international market, as well the establishment and enforcement of protective laws, allowed recuperation of
*Corresponding author. Email: [email protected]
ISSN 0165-0521 print/ISSN 1744-5140 online
© 2012 Taylor & Francis
http://dx.doi.org/10.1080/01650521.2012.662795
http://www.tandfonline.com
some giant-otter populations, noticeably in Colombia
(Díaz & Sánchez 2002), Peru (Uscamaita & Bodmer
2009), Bolivia (Van Damme et al. 2002), and Brazil
(Schweizer 1992; Ribas 2004; Leuchtenberger &
Mourão 2008; Rosas et al. 2008). In Brazil, well
established giant-otter populations are limited to the
Amazonian and Pantanal basins, and in the latter,
groups are widely distributed along the rivers and
creeks of the floodplain (Schweizer 1992; Carter &
Rosas 1997; Ribas 2004; Tomas et al. 2011).
We present the first record of giant-otter predation
on caimans in the Pantanal, which is likely to be due
to food limitation of peripheral populations of giant
otters that have been excluded from larger rivers by
other groups. We also describe behaviors which indicate competitive interactions among group members,
rather than cooperative hunting of a preferred prey.
Materials and methods
Study area
Pantanal, the largest South American wetland,
covers 160,000 km2 of lowland terrain extending
from Bolivia to Paraguay and Brazil. The Pantanal is
20
C. Ribas et al.
recognized by its flatness and low altitude, and suffers
alternating periods of flood and drought with strong
seasonality (Alho et al. 1988; Harris et al. 2005). The
Estrada-Parque Pantanal (EPP) highway is a 120 km
dirt road constructed on a causeway 1–2 m above the
surrounding plains across a section of the southern
Pantanal. It crosses a diverse range of water bodies
and marshlands, including streams, lakes, and ponds
along the roadside, and trenches originating from the
construction of the road (borrow pits). In the dry
season (June to December), the availability of water
gradually diminishes, and much of the aquatic fauna is
restricted to shallow pools in sub-optimal habitat until
the next rainy season.
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Monitoring of giant otter behavior and vocalization
We started monitoring giant otters in the southern
Pantanal (Mato Grosso do Sul, Brazil) in 2002, and
we became aware of marginal populations of giant
otters in pools and water bodies around the EPP
highway (UTM 21k 0451800 7873700 to 21k 0496300
7831500) in 2008. We do not have records of giant
otters occupying borrow pits along roads during the
dry season before then. We recorded behavior of
giant otters with a digital camera (Sony model DCRTRV340), and used the sound-tracks of the records to
extract the sounds produced by the animals in digital
format. Spectrogram and analysis of acoustic variables
(frequency of highest energy [FM], and time duration of sound [DT]) were performed using Bioacustic
Raven 1.2 (Cornell Lab of Ornithology, Ithaca, NY,
USA), and the vocalizations were classified according
to Duplaix (1980).
Results
We accompanied a group of giant otters from October
to December 2009. In the first month, the five adults
that composed the group changed their territory and
traveled about 6 km through savannas, drying lakes
and creeks. In early November, the group won a fight
against another large group and conquered a lake of
about 0.2 km2 , incorporating a male and a female of
the eight individuals from the other group. During
October and November, they preyed almost only on
small loricariid catfish (< 20 cm), characteristic of
benthic environments. By the end of November, most
fish were scarce and the otters dug deep into the mud
to catch marbled swamp eels (Synbranchus marmoratus, Synbranchidae). In December, at the end of the
dry season, we twice registered the group feeding on
large sub-adult caimans (Caiman crocodilus yacare,
Alligatoridae) estimated to be 1.2 and 1.4 m total
length, with an estimated mass of 6 kg and 9 kg.
The caimans were captured by individual giant otters,
although more than one giant otter was present during
the captures. On each occasion, the giant otters began
feeding on the back of the caiman’s neck (Figure 1).
Capture and consumption of the first caiman lasted
53 minutes. The giant otters did not voluntarily share
the prey with other members of the group. However,
in the first event three individuals fed on the same
caiman, which was successively stolen after vocal
and/or physical conflicts. The subordinate that hunted
and first fed on the caiman, vocalized a growl sound
(n = 19) when other adults approached him and his
prey. The growls had an average FM of 465.6 Hz (±
267.7) and an average DT of 2.25s (± 1.33) (Figure 2a).
During the last 6 minutes of feeding, the dominant
male approached the second subordinate feeding on
the caiman and produced long wavering screams (n =
37; FM = 2411.9 kHz ± 492.39; DT = 0.96 s ± 1.23),
apparently soliciting the prey. The average DT of these
screams was 1.49 s (± 1.39), at times interspersed with
the HAH call (Duplaix 1980) of a higher frequency
(n = 23; FM = 3067.29 kHz ± 520.94; DT = 0.54 s ±
0.79) (Figure 2b). Vocalization lasted until the theft of
the prey.
When we made these observations, the individuals in the group were visibly debilitated, thin, and
with the mucous membranes of their anus, mouth and
eyes apparently inflamed. One of the lower canines of
the dominant male was missing and his carnassials
were visibly worn, probably due to the friction with
the sand and mud. Despite the fact that they could
catch caimans, which were common in the area, we
noted that the overall health of the group deteriorated as the dry spell intensified. The otters had taken
caimans before. Analysis of feces from the group’s
latrine collected on November 19 revealed the presence of osteoderms and other remains of caimans,
and local residents reported that giant otters had
been seen feeding on caiman along the highway since
November.
Discussion
Otters, in general, are highly specialized carnivores with a diet dominated by fish (Kruuk 2006).
Nevertheless, there is evidence that high levels of
water instability lead to decreased fish consumption
by Eurasian otters (Clavero et al. 2008; Román 2010).
In eight years and 3000 h of giant-otter observation on
rivers with some hydrological predictability, we have
seen consumption of prey other than fish only three
times, and those prey were crabs (Trichodactylidae).
Even in optimum conditions, caimans compete
with giant otters for fish and can be potential predators of their cubs (Schweizer 1992). During the
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Studies on Neotropical Fauna and Environment
21
Figure 1. A subordinate giant otter (a) brings the still alive caiman to the shore, (b) crosses the highway, (c) begins feeding on
the back of the caiman’s neck and (d) eats the caiman.
last two years we registered 12 agonistic encounters
between yacare caiman and giant otters in the southern Pantanal. Giant otters generally surround and
fence off caimans that get too close to their den
entrances and campsites. In six instances, only one
giant otter was the aggressor, despite the presence of
other members of group. Only once a caiman initiated
the aggression, attacking a giant otter about to leave
water to go to a latrine. The otters never killed or ate
caimans in these cases.
Brecht-Munn and Munn (1988) reported that
giant otters hunting cooperatively can kill large prey
including anacondas (Eunectes sp.) and juvenile black
caimans (Melanosuchus niger). This did not occur in
our observations; each group member hunted its own
prey and did not share the prey voluntarily, as do other
social carnivores that have cooperative hunting, such
as lions and wild dogs (Stander 1992; Creel & Creel
1995).
Because of their high metabolic rate, giant otters
need to eat up to 3 kg of fish per day to maintain
their activities (Duplaix 1980; Kruuk 2006). By the end
of the dry season, the quantity of fish in the borrow
pits along roads is drastically reduced, mainly from
the high rate of predation by piscivorous animals, such
as birds and caimans. At the same time, the densities
of caimans increases as they congregate in diminishing
water pools in the flood plains.
Our data suggest that predation on caimans by
giant otters along the Estrada Parque Pantanal highway was due to the drought and resource scarcity in
these marginal environments. That some groups are
occupying sub-optimal habitats, and feeding on prey
that normally is not taken, is probably related to the
increase in otter density in the Pantanal and suggests
that giant otter populations might be reaching their
carrying capacity in the southern Pantanal. As otter
density increases, and more groups are pushed into
marginal habitat, it is likely that their interactions
with other species will increase in ways not predictable
from data collected from low-density populations in
primary habitat.
C. Ribas et al.
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22
Figure 2. Spectrograms of giant otter sounds vocalized during caiman predation; (a) growl, (b) wavering scream interspersed
with the HAH call (arrow).
Acknowledgments
We thank Federal University of Mato Grosso
do Sul to provide logistical support. This study
formed part of the INPA’s Ph.D. dissertation, which
received funding from the CNPq (scholarship and
grant # 476939/2008-9). We thank Isac Teixeira de
Carvalho for assistance in the field, and Dr. Leslie
Damasceno for revision of the English expression
of the manuscript. We are especially grateful to
Patrícia Medici for inspiring our research in the
Estrada Parque Pantanal. We also acknowledge the
two reviewers who presented relevant comments on
the manuscript.
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